We had a 10 inch diameter metal chimney duct running from the basement to the roof. After replacing the furnace with a direct vent one it was no longer used. Cue more renovations and the section on the first floor was removed. After a new roof was installed that removed the chimney assembly and all we have left is a section hidden behind a wall on the second floor.

Our problem is that there is heat loss going on. Enough so that snow melts on the roof right under where the old chimney enters the attic. I went up to the attic and stuffed the chimney duct with fibreglass and tried to cap and seal it but this has only diminished, not removed, the problem.

If I broke through the drywall to expose the only section of chimney duct left on the second floor what is the most efficient way to seal the area after removing the duct?

I could cut out some Styrofoam SM or similar impervious insulation and spray foam around the edges at the top and bottom.

1 Answer
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It sounds like you have a convective loop, in which case the reason why the fiberglass insulation was not as effective as you would have liked is because fiberglass is air-permeable. Since you have access to the duct itself, try sealing it off with a section of fitted and caulked insulation foam board or something. Doesn't matter where in the duct you do it; the important part is to stop the movement of air to break the loop.